<!-- <script>
  var a = 0

  if (a > 0) {
    alert('a是大于0的')
  } else if (a < 0) {
    alert('a是小于0的')
  } else {
    console.log('a的值是0');
  }

</script> -->
<script>
  var a = 1

  switch(a) {
    case 1:
      alert('111')
      break
    case 2: 
      alert('222')
      break
    default: 
      alert('都不是')
  }

</script>